1. Field of Invention
The present invention relates to image processing systems and methods having a capability for processing and reproducing halftone original images and more particularly to systems and methods for descreening halftone images without screen structure, and well as other halftone images, to continuous tone images with increased quality and efficiency.
2. Description of Related Art
Most printing devices are based on binary technologies. That is, they print using ON/OFF, ink or non-ink printing. To simulate intermediate colors or gray levels, halftoning is used. Halftoning involves use of patterns or ink dots at a high spatial frequency to simulate to the eye an integrated gray scale image. Halftoning techniques are also used to render electronic color images on common electronic display devices that use color mapping to limit the number of bits per pixel and thereby reduce memory requirements.
The use of halftoning can, however, cause problems if a printed copy of the image is desired. This is particularly problematic when using a digital copier. When copying, the image is typically scanned and then printed. The printer has its own halftone pattern that can interfere with the original halftone to produce low-frequency moire patterns that can be objectionable to the viewer of the printed copy. Similar problems can occur if printed copies of an electronic image originally intended for display are desired. In particular, it is possible that the original halftone may be much easier to see in print than on display and may be objectionable to the viewer.
Thus, it is difficult to process halftone images. Therefore, a halftone is often converted to a continuous tone image to enable processing and then reconverted to halftone for printing.
Image processing systems used with printers in reprographic systems typically require a capability for converting halftone images to continuous tone images to meet reconversion needs and for converting scanned halftone images to continuous tone images that can then be processed by any of a large variety of enhancement algorithms commonly available for continuous tone images.
The halftoning process loses some image information in the conversion of the original continuous tone image to a halftone image. The reconversion of a halftone image to a continuous tone image accordingly is essentially an estimation process since the halftoning process cannot be reversed exactly to reproduce a continuous tone image identical to the original image.
The majority of images currently processed in the printing industry are converted from continuous tone to halftone using an ordered dithering method because most printers can only print dithered images. Generally, ordered dithering is a process in which a scanned continuous signal from a continuous tone image is converted to a series of black (1 or ink) or white (0 or no ink) pixels with the pixel values determined by the pattern of a threshold or dither matrix to which the scanned signal is applied.
Another process used to convert continuous tone images to halftone images is called error diffusion. No special thresholding matrix is used in the error diffusion process. Instead, a single threshold is applied to the whole image. Generally, image pixels are processed sequentially, i.e., the first pixel is made either 1 or 0 according to whether its gray level is above or below a predetermined threshold value such as 0.5. The first pixel error is then carried forward and added to the gray value of the unprocessed surrounding pixels in determining whether these pixels are above or below the threshold value. The resultant errors are then carried forward, and the process is continued until the image is completely processed. These halftone screens are typically removed as part of a conversion to continuous tone by low-pass filtering. This removes the high-frequency halftone pattern, but also removes the high frequency information (such as sharp edges) from the image. This results in a picture that looks blurred.
Most attempts at avoiding the loss of image information have been experimented with using halftone originals that are structured, such as those created from ordered dithering patterns. However, until now, such methods were not sufficiently successful when a random-dot halftoning method, such as error diffusion, or a stochastic method, such as blue-noise masking, was used.
The classic prior art method for converting halftone images to continuous tone images, i.e., for “unscreening” continuous tone images from halftone images, applies a low-pass filter to the halftone image data. The low-pass filter method by its nature typically blurs image edges or at least loses fidelity of edge information (fine detail) as a result of the filter conversion process.
U.S. Pat. No. 4,630,125 to Roetling, and assigned to the present assignee, discloses a method of reconstructing a continuous tone image for grayscale values that have been converted to a halftone image of black and white spots. The reconstruction method involves isolation of each spot of a halftone image along with a neighborhood of surrounding spots, and, for each neighborhood, comparing a maximum screen pattern value producing a white spot with a minimum screen value producing a black spot. If the minimum screen value giving a black spot is greater than the maximum screen value giving a white spot, then the grayscale pixel value of the isolated spot is the average of the maximum and minimum screen values just described. If the minimum screen value giving a black spot is less than the maximum screen value giving a white spot, then the process is repeated after deleting that portion of the neighborhood of surrounding spots containing the maximum or minimum screen value furthest from the isolated spot. Use of the Roetling scheme is limited to orthographic or digitally created and stored dithered images since it is based on the regularity of dots in a half-tone image created with a dither.
Another U.S. Pat. No. 4,841,377 issued to Hiratsuka et al. discloses a method for estimating an original continuous tone image from a stored binary image. The method involves, inter alia, setting a plurality of scanning apertures in a binary image formed of a dither matrix, selecting one scanning aperture satisfying a predetermined condition for each picture element of a continuous image to be estimated, and estimating the continuous image on the basis of the number of white or black picture elements in the scanning aperture selected. The Hiratsuka method is similarly limited to dithered halftone images.
More recently, U.S. Pat. No. 5,027,078, issued to the present inventor, Z. Fan, discloses a method for converting halftone images to continuous tone images. The Fan method is an improvement over the Roetling method through the application of “logic filtering.” This logic-filter method provides best results for digitally created and stored halftone images but it is also limited to dithered halftone images.
A prior attempt at reduction of blurring of random halftone dots can be found in U.S. Pat. No. 5,243,444 to Fan, assigned to the same assignee as the present invention and incorporated herein in its entirety. This method first applies a small initial blurring and then iteratively applies Sigma filters. This method, however, can be slow because the Sigma filters must be calculated (and recalculated) for each pixel. This method also suffers from some initial blurring as the Sigma filter is applied to the initially blurred image.
Another attempt can be found in U.S. Pat. No. 5,343,309 to Roetling, assigned to the same assignee as the present invention and incorporated herein in its entirety. This method employs low-pass filtering to generate a first approximation image (FAI). A control then applies an adaptive filter to each pixel of an image as a function of an associated pixel spatial gradient. Then, a number of iterations of filtering are performed, using the output from a previous iteration an input for the subsequent iteration to generate a continuous tone image. However, this requires an iterative process that is time inefficient.
In summary, the prior art generally has had shortcomings in preserving edge smoothness and avoiding edge blur during the “unscreening” of halftone images into contone images. Further, the solutions that did have limited success in reducing edge blurring were very time and calculation intensive, as multiple filtering iterations were necessary.